Multi-thread bone screw and method

ABSTRACT

A bone screw comprises a threaded shank including a distal end portion and a proximal end portion, and defining a first threaded section extending from the distal end portion toward the proximal end portion and adapted for anchoring in cancellous bone. A second threaded section extends contiguously from the first threaded section toward the proximal end portion The second threaded section has a finer thread pattern relative to the first threaded section. In one embodiment, the first threaded section includes a first helical threading defining a single lead thread pattern for anchoring in cancellous bone, and the second threaded section includes a second helical threading interleaved with the first threading to define a duel lead thread pattern for engagement in cortical bone. In a further embodiment, the bone screw includes a head portion extending from the threaded shank and configured for coupling to a spinal implant.

This application is a continuation of co-pending U.S. application Ser.No. 11/355,877, filed Feb. 16, 2006, the entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of bone screws, andmore particularly relates to a bone screw having multiple threadedsections adapted for engagement with different regions of bone, and amethod for using the same.

BACKGROUND

Various types of fasteners are used to engage implants and other devicesto bone. In the spinal field, bone screws are commonly used to attachplates, rods and other types of implants and devices to one or morevertebrae. Referring to FIG. 1, shown therein is a prior art bone screw10 including a threaded shank portion 12 adapted for engagement in bone,and a head portion 14 for coupling to an elongate member (not shown),such as a spinal rod, via a connector mechanism (not shown). Examples ofconnector mechanisms suitable for coupling a spinal rod to the headportion 14 of the bone screw 10 are illustrated and described, forexample, in U.S. Pat. No. 5,643,263 to Simonson, U.S. Pat. No. 5,947,967to Barker and U.S. Pat. No. 6,471,703 to Ashman.

The threaded shank portion 12 of the bone screw includes a single,constant pitch threading 16. The threading 16 comprises a relativelywide pitch that is particularly suitable for engagement or purchasewithin cancellous bone, such as the cancellous bone within the interiorregion of a vertebral body. Although the threading 16 may be providedwith finer pitched threads to increase stability within the relativelyharder and denser cortical region of the bone, finer pitched threadstend to decrease purchase within the cancellous region of the bone.Furthermore, finer pitched threads require additional turns to fullyengage the bone screw within the bone.

Thus, there remains a need for an improved bone screw and a method forusing the same. The present invention satisfies this need and providesother benefits and advantages in a novel and unobvious manner.

SUMMARY

The present invention relates generally to a bone screw having multiplethreaded sections adapted for engagement with different regions of bone,and a method for using the same. While the actual nature of theinvention covered herein can only be determined with reference to theclaims appended hereto, certain forms of the invention that arecharacteristic of the preferred embodiments disclosed herein aredescribed briefly as follows.

In one form of the present invention, a bone screw is provided having athreaded shank including a distal end portion and a proximal endportion, with the threaded shank defining a first threaded sectionextending from the distal end portion toward the proximal end portionand adapted for anchoring in cancellous bone, and with the threadedshank defining a second threaded section extending contiguously from thefirst threaded section toward the proximal end portion and adapted forengagement in cortical bone, and wherein the second threaded sectioncomprises a finer thread pattern relative to the first threaded section.

In another form of the present invention, a spinal system is providedincluding a bone screw with a threaded shank having a distal end portionand a proximal end portion and a head portion extending from theproximal end portion of the threaded shank, with the threaded shankdefining a first threaded section extending from the distal end portiontoward the proximal end portion and adapted for anchoring in cancellousbone, and with the threaded shank defining a second threaded sectionextending contiguously from the first threaded section toward theproximal end portion and adapted for engagement in cortical bone, andwherein the second threaded section comprises a finer thread patternrelative to the first threaded section. The spinal system furtherincludes a spinal implant coupled to the head portion of the bone screw.

In a further form of the present invention, a bone screw is providedhaving a threaded shank including a distal end portion and a proximalend portion, with the threaded shank including a first helical threadingextending from the distal end portion toward the proximal end portionand defining a single lead thread pattern adapted for anchoring incancellous bone, and with the threaded shank including a second helicalthreading interleaved with the first threading to define a duel leadthread pattern adjacent the proximal end portion of the threaded shankadapted for engagement in cortical bone.

In still another form of the present invention, a method is provided forengaging a bone screw to a bone having an inner cancellous region and anouter cortical bone region. The method includes the step of providing abone screw having a threaded shank including a distal end portion and aproximal end portion, with the threaded shank defining a first threadedsection extending from the distal end portion toward the proximal endportion and adapted for anchoring in cancellous bone, and with thethreaded shank defining a second threaded section extending contiguouslyfrom the first threaded section toward the proximal end portion andadapted for engagement in cortical bone, and wherein the second threadedsection comprises a finer thread pattern relative to the first threadedsection. The method further includes the steps of engaging the firstthreaded section within the inner cancellous region of the bone, andengaging the second threaded section within the outer cortical region ofthe bone.

It is one object of the present invention to provide an improved bonescrew and a method for using the same. Further objects, features,advantages, benefits, and aspects of the present invention will becomeapparent from the drawings and description contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational side view of a prior art bone screw.

FIG. 2 is an elevational side view of a bone screw according to one formof the present invention.

FIG. 3 is an elevational side view of the bone screw illustrated in FIG.2, as engaged to bone and as coupled to a spinal rod.

FIG. 4 is an elevational side view of a bone screw according to anotherform of the present invention, as engaged to bone and as coupled to aspinal rod.

FIG. 5 is an elevational side view of a bone screw according to anotherform of the present invention, as engaged to bone and as coupled to aspinal plate.

FIG. 6 is an elevational side view of a bone screw according to anotherform of the present invention, as engaged to bone and as coupled to aspinal plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is hereby intended, and that alterations and furthermodifications to the illustrated devices and/or further applications ofthe principles of the invention as illustrated herein are contemplatedas would normally occur to one skilled in the art to which the inventionrelates.

Referring to FIG. 2, shown therein is a bone screw 50 according to oneform of the present invention. The bone screw 50 extends along alongitudinal axis L and includes a distal end portion 50 a and aproximal end portion 50 b. The bone screw 50 generally includes athreaded shank portion 52 adapted for engagement within bone, and a headportion 54 adapted for coupling with an implant, further details ofwhich will be set forth below. The bone screw 50 may be formed of anysuitable biocompatible material such as, for example, titanium, atitanium alloy, stainless steel, metallic alloys, or other materialsknown to those of skill in the art that possess the mechanical andbiocompatible properties suitable for implantation within the body andattachment to bone.

In one embodiment, the threaded shank 52 includes a distal tip 56 thatis configured to penetrate bone. In the illustrated embodiment, thedistal tip 56 is tapered or pointed to facilitate entry into bone.However, in other embodiments, the distal tip 56 may define a blunt orrounded end. In further embodiments, the distal tip 56 or other portionsof the distal end portion 50 a may be provided with one or more cuttingedges or flutes (not shown) to provide the bone screw 50 withself-cutting or self-tapping capabilities. In still other embodiments,the bone screw 50 may be provided with an axial passage (not shown)extending from the proximal end portion 50 b and partially or entirelytherethrough to define a cannulation opening, and may be furtherprovided with transverse passages that communicate with the axialpassage to define fenestration openings. The cannulation andfenestration openings may be used to deliver material such as, forexample, bone cement from the proximal end portion 50 b of the bonescrew 50 and into areas of the bone axially or laterally adjacent thedistal end portion 50 a or other portions of the threaded shank 52.

In the illustrated embodiment, the head portion 54 comprises arelatively smooth shaft or stem 58 configured to slidably receive animplant member or a connector for coupling to an implant member.However, it should be understood that other types and configuration ofscrew head portions 54 are also contemplated, several examples of whichwill be discussed below. Additionally, bone screw embodiments are alsocontemplated which do not include a screw head portion. The bone screw50, and particularly the screw head portion 54, preferably includesfeatures that allow for releasable engagement with a driving tool orinstrument (not shown) such as, for example, a screwdriver. In oneembodiment, the screw head portion 54 may define a cavity or recess (notshown) sized and shaped to receive a distal end portion of a drivertool. The cavity or recess is preferably non-circular such as, forexample, hexagonal or rectangular shaped to provide non-rotationalengagement between the head portion 54 and the driver tool to facilitatedriving engagement of the bone screw 50 into bone. Alternatively, thescrew head portion 54 may define external surface features forengagement by the distal end portion of a driver tool.

The threaded shank 52 has an overall length l and defines a firstthreaded section 60 extending along a first shank length l₁ from thedistal end portion 50 a toward the proximal end portion 50 b, and asecond threaded section 62 extending contiguously from the firstthreaded section 60 to the proximal end portion 50 b along a secondshank length l₂. As will be discussed in greater detail below, the firstthreaded section 60 includes a first threading 64 that is adapted foranchoring in the cancellous region of a bone, and the second threadedsection 62 includes a relatively finer threading 66 that is adapted foranchoring in the cortical region of the bone. Additionally, the secondthreaded section 62 may define a thread run out 68 adjacent the proximalend portion 50 b of the threaded shank 52.

In one embodiment, the length l₁ of the first threaded section 60extends along at least about one half of the overall length l of thethreaded shank 52, with the length l₂ of the second threaded section 62extending along the remainder of the overall shank length l. In anotherembodiment, the length l₁ of the first threaded section 60 extends alongat least about two-thirds of the overall length l of the threaded shank52, with the length l₂ of the second threaded section 62 extending alongthe remainder of the overall shank length l. In a further embodiment,the length l₁ of the first threaded section 60 extends along at leastabout three-quarters of the overall length l of the threaded shank 52,with the length l₂ of the second threaded section 62 extending along theremainder of the overall shank length l. However, it should beunderstood that other lengths l₁ of the first threaded section 60relative to the overall length l of the threaded shank 52 are alsocontemplated as falling within the scope of the present invention.

As should be appreciated, the particular ratio between the shank lengthsl₁, l₂ associated with the first and second threaded sections 60, 62should preferably be selected based on the characteristics of the boneto which the bone screw 50 is to be engaged. As discussed above, thefirst threaded section 60 includes thread features that are particularlysuited for anchoring in the cancellous region of bone, and the secondthreaded section 62 includes thread features that are particularlysuited for anchoring in the cortical region of bone. In order tomaximize the anchoring effectiveness of the bone screw 50, the lengthsl₁, l₂ of the first and second threaded sections should preferablycorrespond to the desired anchoring depth within the cancellous regionof bone and the thickness of the cortical region of bone, respectively.

In the illustrated embodiment of the invention, the first threading 64includes a single lead in the form of a helical thread pattern whichdefines the first threaded section 60, and the second threading 66cooperates with the first threading 64 to provide a dual lead in theform of double helical thread pattern which defines the second threadedsection 62. In the illustrated embodiment, the first and secondthreadings or leads 64, 66 are provided in the form of a helix thatextends substantially continuously about the longitudinal axis L andalong the length of the shank portion 52. Because the second threading66 is preferably uniformly and centrally offset relative to the firstthreading 64, the threadings 64, 66 appear to spiral together along thelength l₂ of the second threaded section 62 as a continuous thread, butwhich in actuality comprise separate threadings. As will be discussedbelow, providing separate threadings along the second threaded section62 allows the thread pitch associated with each of the first and secondthreaded sections 60 and 62 to be equal if so desired.

In one specific embodiment, the first threading 64 has a first threadpitch p₁, and the second threading 66 is interleaved with the firstthreading 64 and has a second thread pitch p₂ that is substantiallyequal to the first thread pitch p₁. As should be appreciated, since thefirst and second threadings 64, 66 cooperate with one another to providea dual lead thread defining the second threaded section 62, and sincethe first and second threadings 64, 66 have a substantially equal threadpitch p, engagement of the second threaded section 62 into bone will notrequire any additional turns relative to the first threaded section 60.Additionally, in the illustrated embodiment, the second threading 66 isoffset about 180 degrees relative to the first threading 64 such thatthe turns of the second threading 66 are substantially centered betweenadjacent turns of the first threading 64. Although a specific threadconfiguration and arrangement has been illustrated with regard to thebone screw 50, it should be understood that other configurations andarrangements of threadings are also contemplated for use in associationwith the present invention.

As should be appreciated, providing the first threading 64 with a singlelead thread having a relatively large thread pitch tends to increasecancellous bone purchase capabilities, while at the same timemaintaining the strength and structural integrity of the cancellousbone. Additionally, providing the second threading 66 with a dual leadthread provides increased fixation strength and stability within therelatively harder and denser cortical bone. Moreover, as indicatedabove, providing the second threaded section 62 with a dual lead threadhaving the same thread pitch p as the first threaded section 60 will notrequire any additional turns of the bone screw 50 for engagement withincortical bone as compared to engagement of the first threaded section 60within cancellous bone.

In another aspect of the invention, the first and second threadedsections 60, 62 define an inner thread root diameter d_(i) that variesbetween the distal end portion 50 a and the proximal end portion 50 b.In one embodiment, the inner thread root diameter d_(i) increases from afirst root diameter d₁ adjacent the distal end portion 50 a, to a largersecond root diameter d₂ adjacent a mid-section of the threaded shank 52,to an even larger third root diameter d₃ adjacent the proximal endportion 50 b. In another embodiment, the inner thread root diameterd_(i) increases uniformly between the root diameter d₁ adjacent thedistal end portion 50 a and the root diameter d₃ adjacent the proximalend portion 50 b. In a further embodiment, the inner thread rootdiameter d_(i) increases along substantially the entire length l of thethreaded shank 52 between the distal end portion 50 a and the proximalend portion 50 b. In one specific embodiment, the inner thread rootdiameter d_(i) increases by at least about 15% between the root diameterd₁ adjacent the distal end portion 50 a and the root diameter d₃adjacent the proximal end portion 50 b. In another specific embodiment,the inner thread root diameter d_(i) increases by at least about 25%between the root diameter d₁ and the root diameter d₃. In a furtherspecific embodiment, the inner thread root diameter d_(i) increases byat least about 50% between the root diameter d₁ and the diameter d₃.

As should be appreciated, increasing the inner thread root diameterd_(i) from the distal end portion 50 a toward the proximal end portion50 b, results in a decrease or reduction in the depth of the first andsecond threadings 64, 66 (as measured from the thread root diameter tothe outer thread diameter) from the distal end portion 50 a toward theproximal end portion 50 b. It should further be appreciated thatproviding the first threading 64 with a relatively large thread depth(or a relatively small root diameter d_(i)) provides increasedcancellous bone purchase capabilities and desirable pulloutcharacteristics compared to the smaller thread depth associated with thesecond threading 66. Additionally, providing the second threading 66with a reduced thread depth (or a relatively larger inner thread rootdiameter d_(i)) relative to the first threading 64 provides increasedfixation strength and stability, which is particularly advantageous forfixation within the relatively harder and denser cortical bone, and alsotends to reduce resistance to threading engagement within the corticalbone. In one specific embodiment, the thread depth of the first andsecond threadings 64, 66 decreases by at least about 50% from the distalend portion 50 a toward the proximal end portion 50 b. As should beappreciated, the thread depth of the threadings 64, 66 is maximizedalong the first threaded section 60, and particularly adjacent thedistal end portion 50 a, to provided increased cancellous bone purchasecapabilities and desirable pullout characteristics.

In another aspect of the invention, the first and second threadedsections 60, 62 define a substantially uniform outer thread diameterd_(o) between the distal end portion 50 a and the proximal end portion50 b. In one specific embodiment, the outer thread diameter d_(o) is atleast about 15% greater than the inner thread root diameter d_(i)adjacent the distal end portion 50 a. In another specific embodiment,the outer thread diameter d_(o) is at least about 25% greater than theinner thread root diameter d_(i) adjacent the distal end portion 50 a.In a further specific embodiment, the outer thread diameter d_(o) is atleast about 50% greater than the inner thread root diameter d_(i)adjacent the distal end portion 50 a. As should be appreciated, a largervariation between the outer thread diameter d_(o) and the inner threadroot diameter d_(i) tends to increase bone purchase capabilities, whichis particularly suitable along the first threaded section 60 that isengaged in cancellous bone.

In a further aspect of the invention, the thread pitch p adjacent thedistal end portion 50 a is equal to or greater than the inner threadroot diameter d_(i) adjacent the distal end portion 50 a. In oneembodiment, the thread pitch p is equal to or greater than the innerthread root diameter d_(i) along substantially the entire length l₁ ofthe first threaded section 60 of the threaded shank 52. In anotherembodiment, the thread pitch p is equal to or greater than the innerthread root diameter d_(i) along substantially the entire overall lengthl of the threaded shank 52.

As indicated above, the head portion 54 of the bone screw 50 ispreferably adapted for coupling with an implant. As illustrated in FIG.3, in one embodiment, the screw head portion 54 comprises an unthreadedstem portion or shaft 58, and the implant comprises an elongate spinalrod R that is coupled to the screw head 54 via a connector mechanism 70.The connector mechanism 70 includes a connector body 72 defining a firstpassage 74 a for receiving the stem portion 58 of the screw head 54, anda second passage 74 b for receiving the spinal rod R. An interfacemember 76 may be positioned between the spinal rod R and the stemportion 58, and a fastener or set screw 78 is threaded through anopening in the connector body 72 and into contact with the spinal rod R,which in turn engages the interface member 76 with the stem portion 58of the screw head 54 to fix the angular relationship between the spinalrod R and the bone screw 50. Further details regarding the connectormechanism 70 and other types of connector mechanisms are illustrated anddescribed, for example, in U.S. Pat. No. 5,643,263 to Simonson, U.S.Pat. No. 5,947,967 to Barker and U.S. Pat. No. 6,471,703 to Ashman, thecontents of each patent reference hereby incorporated by reference inits entirety.

Referring to FIG. 4, shown therein is a bone screw 80 according toanother form of the invention for coupling with a spinal rod R.Specifically, the bone screw 80 includes a threaded shank 52 and a head84 that is adapted for coupling with a spinal rod R. The threaded shank52 is identical to that described above and illustrated in FIGS. 2 and3. However, the bone screw head 84 defines a U-shaped channel 86 that issized to receive the spinal rod R therein, with the spinal rod Rcaptured within the channel 86 via a fastener or set screw 88 engagedwith the bone screw head 84.

Referring to FIG. 5, shown therein is a bone screw 90 according toanother form of the invention which is adapted for coupling with animplant, such as a spinal plate P. In one embodiment, the spinal plate Pincludes one or more openings 92 for receiving one or more of the bonescrews 90, with the bone screw 90 including an enlarged head portion 94.In the illustrated embodiment, the opening 92 includes a lower portionsized to receive the threaded shank 52 therethrough, and an upperportion sized to receive the enlarged bone screw head 94. As should beappreciated, threading of the bone screw 90 into the bone compresses thespinal plate P against an outer surface of the bone, thereby capturingthe spinal plate P between the enlarged head 94 and the outer surface ofthe bone.

Referring to FIG. 6, shown therein is a bone screw 90′ according to afurther form of the invention for coupling with a spinal plate P′.Specifically, the bone screw 90′ includes a threaded shank 52, anenlarged head portion 94′, and a threaded stem portion 96 extending fromthe enlarged head 94′, with the spinal plate P′ including one or moreopenings 92′ for receiving the threaded stem portion 96 of one or moreof the bone screws 90′ therethrough, and with the spinal plate P′secured to the bone screw 90′ via a lock nut 98 threaded onto the stemportion 96, thereby capturing the spinal plate P between the enlargedhead 94′ and the lock nut 98. As should be appreciated, the bone screw90′ illustrated in FIG. 6 allows the spinal plate P′ to be spaced fromthe outer surface of the bone.

Having described the components and features associated with the presentinvention, reference will now be made to a method for engaging the bonescrew to bone according to one form of the invention. As shown in FIGS.3-6, in one embodiment, the bone comprises a vertebral body V having aninner cancellous bone region 100 and an outer cortical bone region 102.As discussed above, the outer cortical bone region 102 of the vertebralbody V is relatively harder and denser compared to the inner cancellousbone region 100. The threaded shank 52 of the bone screw is driven intoengagement with the vertebral body V, with the first threaded section 60including the single lead threading 64 engaged within the innercancellous bone region 100, and with the relatively finer secondthreaded section 62 including the dual lead threading 66 engaged withinthe outer cortical bone region 102.

As should be appreciated, the relatively course single lead threading 64provides increased bone purchase capabilities and desirable pulloutcharacteristics compared to the finer dual lead threading 66, which isparticularly advantageous for anchoring within soft cancellous bone. Asshould be further appreciated, the relatively fine dual lead threading66 provides increased fixation strength and stability compared to thecourser single lead threading 64, which is particularly advantageous forfixation within relatively harder and denser cortical bone.Additionally, since the single lead threading 64 and the dual leadthreading 66 each have substantially equal thread pitches p₁, p₂,threading of the second threaded section 62 into the cortical bone 102will not require any additional turns relative to threading of the firstthreaded section 60 into the cancellous bone 100 to fully engage thebone screw within the vertebral body V. As should be appreciated aspinal implant such as a rod or plate may be coupled to one or more bonescrews which are in turn anchored to corresponding vertebral bodies Vfor treatment of the spinal column.

It should be understood that the bone screws of the present inventionmay be anchored within any number of vertebral bodies V, including asingle vertebral body or two or more vertebral bodies. In one embodimentof the invention, the bone screws are anchored within the pedicle regionof a vertebral body. However, it should be understood that the bonescrews may be anchored to other portions or regions of a vertebral body.It should also be understood that the bone screws of the presentinvention may be anchored to a posterior, anterior, lateral,posterolateral or anterolateral aspect of the vertebral body V. Itshould further be understood that the bone screws of the presentinvention may be attached to any region of the spinal column, includingthe cervical, thoracic, or lumbar regions of spinal column. It shouldlikewise be understood that the bone screws of the present invention maybe attached to bone structures other than vertebral bodies, such as, forexample, bones associated with the arm or leg.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments have been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. A bone screw configured to enter into tissuehaving a first hardness and a second hardness, the bone screwcomprising: a threaded shank extending along a longitudinal axis betweena distal end portion and a proximal end portion, said threaded shankdefining a first threaded section extending from said distal end portiontoward said proximal end portion, said threaded shank defining a secondthreaded section extending contiguously from said first threaded sectiontoward said proximal end portion, said second threaded sectioncomprising a finer thread pattern relative to said first threadedsection, wherein said threaded shank includes a first helical threadinghaving a single thread type extending along said threaded shank fromsaid first threaded section and into said second threaded section, saidthreaded shank including a second helical threading interleaved withsaid first helical threading to define said second threaded section,each of said first and second helical threadings having a substantiallyequal thread pitch, a head portion extending from said proximal endportion of said threaded shank, said bone screw being coaxial with saidlongitudinal axis from said proximal end portion to said head portion,said head portion being configured to have a spinal implant coupledthereto, wherein said head portion comprises a pair of spaced apart armsdefining a U-shaped channel therebetween and said spinal implantcomprises a spinal rod that is configured to be captured within theU-shaped channel, and a set screw having a threaded outer surface,wherein inner surfaces of said arms are threaded and are configured toengage the threaded outer surface to couple said set screw to said headportion, wherein said bone screw is configured for insertion into aninner region of said tissue having said first hardness and an outerregion of said tissue having said second hardness, said second hardnessbeing greater than said first hardness, said bone screw being configuredfor insertion into said tissue such that said first threaded sectionpenetrates said inner region after said first threaded sectionpenetrates said outer region.
 2. The bone screw of claim 1, wherein:said proximal end portion is configured to engage said outer regionwithout moving through said inner region; and said distal end portion isconfigured to move through said outer region and engage said innerregion.
 3. The bone screw system of claim 1, wherein said first threadedsection has a larger thread depth than said second threaded section. 4.The bone screw system of claim 3, wherein said first and second threadedsections define a substantially uniform outer thread diameter betweensaid distal end portion and said proximal end portion such that saiddistal end portion has a greater thread depth than said proximal endportion.
 5. The bone screw of claim 1, wherein: said second threadedsection is configured to engage said inner region; and said firstthreaded region is configured to be positioned completely within saidouter region when said second threaded region engages said inner region.6. The bone screw system of claim 1, wherein said threaded shank has aninner thread root diameter that increases continuously from said distalend portion to said proximal end portion.
 7. The bone screw of claim 1,wherein said second helical threading includes a buttress thread form.8. The bone screw of claim 1, wherein said distal end portion comprisesone or more cutting flutes to provide the bone screw with self-tappingcapabilities.
 9. The bone screw of claim 1, wherein said bone screw iscoaxial with said longitudinal axis from said distal end portion to saidproximal end portion.
 10. A bone screw configured to enter into tissuehaving a first hardness and a second hardness, the bone screwcomprising: a threaded shank extending along a longitudinal axis betweena distal end portion and a proximal end portion, said threaded shankcomprising a middle portion extending between said distal end portionand said proximal end portion; a first helical buttress threadingextending continuously from said distal end portion to said proximal endportion of said threaded shank; a second helical buttress threadinginterleaved in said first helical threading extending from said middleportion to said proximal end portion such that said second helicalthreading does not extend to said distal end portion, said first andsecond helical threadings having a substantially similar thread pitch; ahead portion extending from said proximal end portion of said threadedshank, said bone screw being coaxial with said longitudinal axis fromsaid proximal end portion to said head portion, said head portioncomprising a pair of spaced apart arms defining a U-shaped channeltherebetween and said spinal implant comprises a spinal rod that isconfigured to be captured within the U-shaped channel; and a set screwhaving a threaded outer surface, wherein inner surfaces of said arms arethreaded and are configured to engage the threaded outer surface tocouple said set screw to said head portion, wherein said bone screw isconfigured for insertion into an inner region of said tissue having saidfirst hardness and an outer region of said tissue having said secondhardness, said second hardness being greater than said first hardness,wherein said bone screw is configured for being inserted into saidtissue such that said first helical threading penetrates said innerregion after said first helical threading penetrates said outer region.11. The bone screw of claim 10, wherein said second threading issubstantially centered between said first threading to define saiddouble helical thread pattern that engages said second region.
 12. Thebone screw of claim 10, wherein said second threading is offset about180 degrees relative to said first threading.
 13. The bone screw ofclaim 10, wherein: said proximal end portion is configured to engagesaid outer region without moving through said inner region; and saiddistal end portion is configured to move through said outer region andengage said inner region.
 14. The bone screw of claim 10, wherein saidsecond threading extends along about one-thirds of said threaded shank.15. A bone screw configured to enter into tissue having a first hardnessand a second hardness, the bone screw comprising: a threaded shankextending along a longitudinal axis between a distal end portion and aproximal end portion, said threaded shank comprising a middle portionextending between said distal end portion and said proximal end portion;a first helical buttress threading extending continuously from saiddistal end portion to said proximal end portion of said threaded shank;a second helical threading extending from said middle portion to saidproximal end portion such that said second helical threading does notextend to said distal end portion, said first and second helicalthreadings having a substantially similar thread pitch; a head portionextending from said proximal end portion of said threaded shank, saidbone screw being coaxial with said longitudinal axis from said proximalend portion to said head portion, said head portion comprising a pair ofspaced apart arms defining a U-shaped channel therebetween and saidspinal implant comprises a spinal rod that is configured to be capturedwithin the U-shaped channel; and a set screw having a threaded outersurface, wherein inner surfaces of said arms are threaded and areconfigured to engage the threaded outer surface to couple said set screwto said head portion, wherein said bone screw is configured forinsertion into an inner region of said tissue having said first hardnessand an outer region of said tissue having said second hardness, saidsecond hardness being greater than said first hardness, said bone screwbeing configured to inserted into said tissue such that said firsthelical threading penetrates said inner region after said first helicalthreading penetrates said outer region.
 16. The bone screw of claim 15,wherein said second threading is substantially centered between saidfirst threading to define said double helical thread pattern thatengages said second region.
 17. The bone screw of claim 15, wherein saidsecond threading is offset about 180 degrees relative to said firstthreading.
 18. The bone screw of claim 15, wherein: said proximal endportion is configured to engage said outer region without moving throughsaid inner region; and said distal end portion is configured to movethrough said outer region and engage said inner region.
 19. The bonescrew of claim 15, wherein said second threading extends along aboutone-thirds of said threaded shank.